3 files changed, 69 insertions, 27 deletions
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index d768f06c4f0..d9faa6777ea 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -616,19 +616,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     match self.size_and_align_of(metadata, &field)? {
                         Some(size_and_align) => size_and_align,
                         None => {
-                            // A field with extern type.  If this field is at offset 0, we behave
-                            // like the underlying extern type.
-                            // FIXME: Once we have made decisions for how to handle size and alignment
-                            // of `extern type`, this should be adapted.  It is just a temporary hack
-                            // to get some code to work that probably ought to work.
-                            if sized_size == Size::ZERO {
-                                return Ok(None);
-                            } else {
-                                span_bug!(
-                                    self.cur_span(),
-                                    "Fields cannot be extern types, unless they are at offset 0"
-                                )
-                            }
+                            // A field with an extern type. We don't know the actual dynamic size
+                            // or the alignment.
+                            return Ok(None);
                         }
                     };
 
diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 851c2a6bb2e..818b95b7fc4 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -362,21 +362,15 @@ where
             // Re-use parent metadata to determine dynamic field layout.
             // With custom DSTS, this *will* execute user-defined code, but the same
             // happens at run-time so that's okay.
-            let align = match self.size_and_align_of(&base.meta, &field_layout)? {
-                Some((_, align)) => align,
-                None if offset == Size::ZERO => {
-                    // An extern type at offset 0, we fall back to its static alignment.
-                    // FIXME: Once we have made decisions for how to handle size and alignment
-                    // of `extern type`, this should be adapted.  It is just a temporary hack
-                    // to get some code to work that probably ought to work.
-                    field_layout.align.abi
+            match self.size_and_align_of(&base.meta, &field_layout)? {
+                Some((_, align)) => (base.meta, offset.align_to(align)),
+                None => {
+                    // For unsized types with an extern type tail we perform no adjustments.
+                    // NOTE: keep this in sync with `PlaceRef::project_field` in the codegen backend.
+                    assert!(matches!(base.meta, MemPlaceMeta::None));
+                    (base.meta, offset)
                 }
-                None => span_bug!(
-                    self.cur_span(),
-                    "cannot compute offset for extern type field at non-0 offset"
-                ),
-            };
-            (base.meta, offset.align_to(align))
+            }
         } else {
             // base.meta could be present; we might be accessing a sized field of an unsized
             // struct.
diff --git a/src/test/ui/consts/const-eval/issue-91827-extern-types.rs b/src/test/ui/consts/const-eval/issue-91827-extern-types.rs
new file mode 100644
index 00000000000..d5576ebfd02
--- /dev/null
+++ b/src/test/ui/consts/const-eval/issue-91827-extern-types.rs
@@ -0,0 +1,58 @@
+// run-pass
+//
+// Test that we can handle unsized types with an extern type tail part.
+// Regression test for issue #91827.
+
+#![feature(const_ptr_offset_from)]
+#![feature(const_slice_from_raw_parts)]
+#![feature(extern_types)]
+
+use std::ptr::addr_of;
+
+extern "C" {
+    type Opaque;
+}
+
+unsafe impl Sync for Opaque {}
+
+#[repr(C)]
+pub struct List<T> {
+    len: usize,
+    data: [T; 0],
+    tail: Opaque,
+}
+
+#[repr(C)]
+pub struct ListImpl<T, const N: usize> {
+    len: usize,
+    data: [T; N],
+}
+
+impl<T> List<T> {
+    const fn as_slice(&self) -> &[T] {
+        unsafe { std::slice::from_raw_parts(self.data.as_ptr(), self.len) }
+    }
+}
+
+impl<T, const N: usize> ListImpl<T, N> {
+    const fn as_list(&self) -> &List<T> {
+        unsafe { std::mem::transmute(self) }
+    }
+}
+
+pub static A: ListImpl<u128, 3> = ListImpl {
+    len: 3,
+    data: [5, 6, 7],
+};
+pub static A_REF: &'static List<u128> = A.as_list();
+pub static A_TAIL_OFFSET: isize = tail_offset(A.as_list());
+
+const fn tail_offset<T>(list: &List<T>) -> isize {
+    unsafe { (addr_of!(list.tail) as *const u8).offset_from(list as *const List<T> as *const u8) }
+}
+
+fn main() {
+    assert_eq!(A_REF.as_slice(), &[5, 6, 7]);
+    // Check that interpreter and code generation agree about the position of the tail field.
+    assert_eq!(A_TAIL_OFFSET, tail_offset(A_REF));
+}